This invention relates to a sintered body of aluminum nitride and method for making.
Sintered alumina bodies have been employed as an insulating substrate for supporting integrated circuits such as silicon chips. However, alumina substrates have a poor thermal conductivity and a higher coefficient of thermal expansion than silicon, resulting in many disadvantages including the poor adhesion of large-sized silicon chips thereto.
One of substituted for alumina is beryllium oxide, which has a thermal conductivity higher than that of alumina by a factor of ten or more. The beryllium oxide, however, is not readily available or handled because it is toxic and expensive.
Silicon carbide substrates were also developed. Production cost is undesirably increased by the use of a hot press in sintering. Also disadvantageously, silicon carbide substrates have a high dielectric constant and a low dielectric strength because silicon carbide is in itself a semiconductor.
Great attention is now paid to sintered bodies of aluminum nitride (AIN) which has a high thermal conductivity and electrical resistance. Aluminum nitride is also characterized by a coefficient of thermal expansion approximate to that of silicon and a low dielectric constant. To take advantage of these attributes, the aluminum nitride sintered bodies must be dense and low in oxygen content.
Since low oxygen content aluminum nitride powder alone can not be sintered well, there is the need for sintering aids capable of promoting sintering. A variety of sintering aids have heretofore been proposed.
For example, one known method is by adding aluminum oxide (Al.sub.2 O.sub.3) or yttorium oxide (Y.sub.2 O.sub.3) to aluminum nitride powder followed by sintering under ambient pressure or hot pressing. Japanese Patent Publication No. 58-49510 discloses a method for sintering aluminum nitride powder in admixture with calcium oxide (CaO), barium oxide (BaO) and/or strontium oxide (SrO) under ambient pressure. Japanese patent application Kokai No. 58-32073 discloses the addition of boron nitride (BN) to aluminum nitride powder. The mixture is sintered in a non-oxidizing atmosphere under ambient pressure or hot pressed. Japanese patent application Kokai No. 59-50077 discloses to add a powder mixture containing at least one member selected from the group consisting of CaO, BaO, and SrO to aluminum nitride powder. The mixture is then hot pressed in a non-oxidizing atmosphere.
Among these prior art techniques, the addition of oxides adversely affects thermal conductivity. The addition of boron nitride is believed favorable among others because the resulting sintered aluminum nitride bodies have a higher thermal conductivity and a higher density.
However, all these conventional sintered bodies of aluminum nitride prepared by any of the above-mentioned methods are disadvantageous in that uneven sintering is likely to occur to such an extent that white spots are sometimes observed on the surface even with the naked eyes. The uneven sintering also results in products having inconsistent electrical resistivity (volume resistivity).
In order to eliminate these disadvantages, there has long been desired to produce an improved sintered body of aluminum nitride using a novel sintering aid.